1. Field of the Invention
The present invention relates to a signal point arrangement dispersion value calculation circuit for calculating a signal point arrangement dispersion value of a baseband signal demodulated from a received phase shift keying signal, and more particularly to a signal point arrangement dispersion value calculation circuit used with a carrier reproduction circuit for reproducing a carrier from a demodulated baseband signal or an absolute phasing circuit for making the phase angle of a baseband signal demodulated from a reception signal point arrangement coincide with a transmission signal phase angle.
2. Description of Related Art
A broadcasting receiver for receiving digitally modulated radio waves such as 8PSK modulated waves, QPSK modulated waves and BPSK modulated waves controls the frequency of a reproduction carrier wave in accordance with a signal point dispersion value of a baseband signal. This technique is disclosed, for example, in Japanese Patent Application No. 9-163530. A phase rotation angle of a current reception signal is obtained from the signal point arrangement of a demodulated baseband signal, and in accordance with the obtained reception signal phase rotation angle, the phase of the demodulated baseband signal is rotated reversely to execute absolute phasing, i.e, to make the demodulation baseband signal coincide with the transmission signal phase angle.
In the case of QPSK modulation, the demodulated baseband signals (I, Q) have reference positions (0, 0), (0, 1), (1, 1) and (1, 0) in which (0, 0) is set in the first quadrant, (0, 1) is set in the second quadrant, (1, 1) is set in the third quadrant and (1, 0) is set in the fourth quadrant, and (0, 1) is rotated by 90xc2x0 in the clockwise direction, (1, 1) is rotated by 180xc2x0 in the clockwise direction and (1, 0) is rotated by 90xc2x0 in the counter-clockwise direction to set all reference positions in the first quadrant. The demodulated baseband signals (I, Q) set in the first quadrant are supplied to a signal point arrangement conversion circuit to convert the signals into signal point arrangement conversion data.
Dispersion values are calculated from the signal point arrangement conversion data converted by the signal point arrangement conversion circuit, and compared with a predetermined reference value A to count the occurrence frequency of dispersion values equal to or larger than the reference value A during a predetermined unit period. A total sum DSMS of occurrence frequencies of dispersion values equal to or larger than the reference value A during the predetermined unit period is calculated. In accordance with this total sum DSMS, a C/N is judged and in accordance with the judged C/N, the frequency of a reproduction carrier is controlled.
According to the above-described conventional technique, the signal point arrangement conversion circuit operates to set the baseband signals: (I, Q) to the first quadrant. In the case of QPSK modulation, setting the baseband signals to the first quadrant is realized by exchanging the I axis or Q axis in each quadrant. In the case of multi-value modulation such as 8 PSK modulation, baseband signals are set to the first quadrant by using a conversion table stored in a ROM.
If the signal point arrangement conversion is performed by using a conversion table stored in ROM, the size of the conversion table stored in ROM becomes too large to be implemented in an integrated circuit.
It is an object of the invention to provide a signal point arrangement dispersion value calculation circuit having a small circuit scale.
A signal point arrangement dispersion calculation circuit of this invention, comprises: a phase rotation circuit for rotating a phase of a demodulated baseband signal; a signal point arrangement conversion circuit for judging a signal point arrangement of the demodulated baseband signal in accordance with the demodulated baseband signal and a baseband signal whose phase is rotated by the phase rotation circuit, and in accordance with the judged signal point arrangement, converting the signal point arrangement of the demodulated baseband signal into a predetermined quadrant by using the demodulated baseband signal and the baseband signal whose phase is rotated by the phase rotation circuit; and dispersion value calculating means for calculating a dispersion value in accordance with the baseband signal whose signal point arrangement is converted.
A signal point arrangement dispersion calculation circuit of this invention, comprises: a phase rotation circuit for rotating a phase of a demodulated baseband signal by 22.5xc2x0 at a speed twice as fast as a symbol rate of the demodulated baseband signal; a signal point arrangement conversion circuit for judging a signal point arrangement of the demodulated baseband signal in accordance with the demodulated baseband signal and a baseband signal whose phase is rotated by the phase rotation circuit, and in accordance with the judged signal point arrangement, converting the signal point arrangement of the demodulated baseband signal into a predetermined quadrant by using the demodulated baseband signal and the baseband signal whose phase is rotated by 45xc2x0 through consecutive two rotations by the phase rotation circuit; and dispersion value calculating means for calculating a dispersion value in accordance with the baseband signal whose signal point arrangement is converted.
According to the signal point dispersion calculation circuit of this invention, the phase rotation circuit rotates rotating the phase of a demodulated baseband signal by 22.5xc2x0 at a speed twice as fast as a symbol rate of the demodulated baseband signal. The signal point arrangement conversion circuit judges a signal point arrangement of the demodulated baseband signal in accordance with the demodulated baseband signal and a baseband signal whose phase is rotated by the phase rotation circuit, and in accordance with the judged signal point arrangement, converts the signal point arrangement of the demodulated baseband signal into a predetermined quadrant by using the demodulated baseband signal and the baseband signal whose phase is rotated by 45xc2x0 through consecutive two rotations by the phase rotation circuit; and the dispersion value calculating means calculates a dispersion value in accordance with the baseband signal: whose signal point arrangement is converted.
The signal point arrangement of the demodulated baseband signal is judged in accordance with the demodulated baseband signal and a baseband signal whose phase is rotated by 22.5xc2x0 by the phase rotation circuit, and in accordance with the judged signal point arrangement, the signal point arrangement of the demodulated baseband signal is converted by using the demodulated baseband signal and the baseband signal whose phase is rotated by 45 through consecutive two rotations by the phase rotation circuit. Accordingly, a conventional conversion table stored in ROM for signal point arrangement conversion is not necessary, and the signal point arrangement conversion circuit can be realized by logic circuits to thus reduce the circuit scale.